For a headlight mounted on a vehicle, in order to improve safety for night driving while expressing excellent design and sophisticated impression, a bright discharge lamp and a light-emitting diode (LEDs) that provide bright illumination in desired directions are widely used as light sources in place of a conventional halogen lamp.
Upon mounting the above-described bright light sources on a vehicle, for example, when the rear of the vehicle drops and leans due to a passenger getting into the rear seat or the loading of luggage into the trunk, in other words, when the front of the vehicle is raised and thus the illumination directions of headlights lean upward, in order to prevent a driver of an oncoming vehicle from getting blinded and also prevent a pedestrian facing the headlights from getting annoyed, there is a need to move the illumination directions of the headlights, i.e., the optical axes of the headlights, downward to maintain the directions of the optical axes with respect to a road surface in fixed directions. In short, for a vehicle that uses the above-described bright light sources, it is necessary to mount an optical axis control device for a headlight thereon. The optical axis control device for a headlight is such that, at least when the vehicle leans due to a passenger getting in the vehicle or the loading of luggage into the trunk and accordingly the illumination directions of headlights are changed upward, the illumination directions of the headlights are operated to move downward to bring the illumination directions back to those before the change.
Note that the passenger's getting in the vehicle or the loading luggage is performed when the vehicle is stopped, and thus, the control of the optical axis by the optical axis control device for a headlight is mainly performed when the vehicle is stopped.
Meanwhile, control of the optical axes of the headlights is to operate the optical axes upward or downward to cancel out a change in vehicle lean angle with respect to a road surface so that the illumination directions of the headlights can be brought back to their original directions when the vehicle leans in the front-rear direction as described above. Thus, first, there is a need to measure the vehicle lean angle with respect to the road surface. In the following, the vehicle lean angle with respect to the road surface is called a “vehicle angle”.
Conventionally, using a stroke sensor mounted on suspensions (suspension devices) at the front and rear of a vehicle, the amounts of compression of the front and rear suspensions, i.e., the amounts of sinking of front and rear axle portions, are measured and a vehicle angle is calculated based on a difference between the amounts of front and rear sinking, and the length of the wheelbase.
In these days, in addition to a configuration using the above-described stroke sensors mounted on suspensions, a configuration has been considered in which an acceleration sensor capable of detecting the gravitational acceleration is used, such as the configuration shown in Patent Literature 1, for example. In the configuration using the acceleration sensor, it is easy to detect a change in the lean angle of a vehicle being stopped, and it is easy to obtain the vehicle angle at the present time by accumulating the amounts of changes occurring due to a passenger getting in and out of the vehicle, etc., on an initial vehicle angle. On the other hand, an offset and a change in the offset over time are present in outputs from the acceleration sensor and the above-described vehicle angle obtained by the accumulation includes a potential cumulative error, so that there is a problem that a vehicle angle obtained by accumulating measured values and changes has low accuracy. Hence, to stably maintain the optical axes of the headlights at proper angles over a long period of time, accuracy needs to be ensured by applying some kind of correction to an acceleration measured by the acceleration sensor or by removing a cumulative error included in a vehicle angle.
An optical axis control device of the above-described Patent Literature 1 increases the accuracy of a vehicle angle while using acceleration sensors for two axes, the front-rear direction and up-down direction of the vehicle, and performs not only optical axis control when the vehicle is stopped, but also optical axis control by measuring an acceleration when the vehicle is travelling, in order to perform preferable control of the optical axes of headlights. The optical axis control device of the above-described Patent Literature 1 obtains, for each period of time, a direction of acceleration change, using an acceleration measured when the vehicle is traveling, or obtains a direction of acceleration change, from two accelerations at different measurement timings, and thereby calculates a vehicle angle and controls the optical axes based on the change in the vehicle angle.